Gemara

(Rav): If a Nazir became Tamei (after the days of Nezirus he accepted) while waiting for his hair to grow (to 30 days growth so he can shave, e.g. his hair was cut against his will), the days he counted are to nullified.

Even R. Yochanan said that Tum'ah nullifies what he counted, only when he is in the midst of Nezirus. Here, he finished his Nezirus!

(Shmuel): This nullifies the days he counted.

Even Reish Lakish said that Tum'ah does not nullify the previous days, only when the previous days were a separate Nezirus. Here, it is all one Nezirus!

46a (Beraisa - R. Eliezer): "V'Achar Yishteh ha'Nazir Yayin" - after all the actions;

Chachamim say, after a single action.

14b (Rav Chisda): All agree that if he became Tamei after bringing the Korbanos but before shaving, he has no solution.

Suggestion: This is like R. Eliezer.

Rejection: R. Eliezer holds that all Isurei Nazir are in effect until he shaves. It is as if he became Tamei within the term. (He has a solution, like any Nazir Tamei!)

Conclusion: Rather, it is like Chachamim.

Question: Chachamim say that Isurei Nezirus go away even if he does not shave!

Answer: Rav Chisda means that he has no solution to perform he Mitzvah of shaving properly (in Taharah, with the Korbanos).

(R. Yosi b'Rebbi Chanina): A Nazir who finished his term (and is ready to bring Korbanos) is lashed if he becomes Tamei, but not for shaving or drinking wine.

Question: Why is Tum'ah different?

Suggestion: "All the days (...he will not become Tamei)" includes the days after the term.

Rejection: It says "all the days" also regarding shaving and drinking wine!

Answer: Tum'ah is different, because it says "he defiled Rosh Nizro." This refers to one whose Nezirus depends on his head (i.e. until he shaves).

Question (Beraisa): A Nazir who finished his term may not shave, drink wine, or become Tamei. If he did any of these, he receives 40 lashes. (The Mitzvah to shave is only after bringing at least one of the Korbanos.)

This refutes R. Yosi b'Rebbi Chanina.

31b (Mishnah): If one accepted Nezirus and he asked a Chacham to permit it, but the Chacham did not, he counts his Nezirus from the day he vowed (even if he was not observing the laws of Nezirus).

Question (Beraisa): If one transgressed his Nezirus, we do not permit his vow until he observes the Isurim for as many days as he transgressed;

R. Yosi says, it suffices to observe the Isurim for 30 days.

Answer #1: Our Mishnah is like R. Yosi. (The case is, at least 30 days of his Nezirus remain.)

Answer #2: The Mishnah is like Chachamim. He does not count Nezirus from the day he accepted, rather, he counts (extra) days like from the day he accepted (and transgressed).

Rishonim

Rambam (Hilchos Nezirus 4:12): If a Nazir finished his term and did not shave Tiglachas Taharah, he may not shave, drink wine, or become Tamei. All laws of Nezirus apply to him. If he shaved, drank, or become Tamei, he is lashed.

Lechem Mishneh: The Rambam means 'he did not bring Korbanos Tiglachas Taharah.' The Rambam (8:5) rules like Chachamim, who say that after one act, i.e. bringing one Korban, the Isurim are permitted.

Minchas Chinuch (368 DH v'Efshar): "Lo Yachel Devaro" does not apply after the term that he accepted. He transgresses only due to a law of Nezirus. The same applies to one who accepted to be a Nazir for only 10 days, or only from wine. "Lo Yachel" applies only to what he said; the other Isurim are due to Nezirus.

Question (Kehilas Yakov, Nazir 2): "Lo Yachel" applies to Nezirus, even though the Ritva (Shevuos 22b DH Oh) says that a Nazir only makes himself a Nazir (Avnei Milu'im Teshuvah 15, citing Maharit - he makes himself Kodesh, somewhat like a Kohen.) The Torah puts all the Isurim on him!

Answer (Kehilas Yakov): Since the Torah obligates a Nazir to observe these laws, if one accepted Nezirus Stam (without specifying) and transgresses, he profanes his word. Sefer ha'Mitzvos (94) obligates one who delays bringing Korbanos that he vowed for "Lo Yachel." This is because one who vows accepts to abide by all Torah laws (Chasam Sofer Nedarim 3B DH Omar). If one explicitly limited his acceptance, and transgresses other matters that the Torah imposes on him, perhaps he did not profane his word!

Rebuttal (Kehilas Yakov DH Emnam): The Rambam says that separate vows join for "Lo Yachel." E.g. he forbade eating figs, i.e. a k'Zayis, and eating grapes, and ate a half k'Zayis of each. This shows that "Lo Yachel" is not for profaning his word, rather, for violating what is forbidden due to his word.

Rambam (13): If one thought that his vow of Nezirus was invalid, and drank wine, and later he heard that it was valid, he counts from when he vowed. Mid'Rabanan, he must observe (extra) days of Isur equal to the days he transgressed.

Rambam (14): E.g. if he vowed for 30 days, transgressed 10 days, and observed 20 days, and after 30 days a Chacham forbade to him, he observes 10 more days. If he shaved, drank, or become Tamei during these 10 days, he is lashed mid'Rabanan.

Merumei Sadeh (Nazir 32a DH Mani): The Rambam discusses after he brought Korbanos. Before bringing Korbanos he is lashed mid'Oraisa, like he said above (Halachah 12).

Question (Keren Orah 32a DH Ayen bi'Lshon): It seems that he must observe the compensatory days before bringing Korbanos. If so, he should be lashed mid'Oraisa!

Note: (Perhaps he holds that had the Rambam discussed one who improperly brought Korbanos too early, he would have specified.)

Achiezer (3:67:4): We can say that the Rambam discusses after bringing Korbanos. However, Nezirus cannot apply afterwards! Perhaps this was the fine, even though it is not applicable.